Recently, a transmission system for automobiles is required to have improved power transmitting efficiency, as well as a small size and a light weight for the demand of improved fuel efficiency of automobiles. The transmission mechanism may be manual or automatic, and recently some automobiles are equipped with a continuously variable transmission.
On the other hand, there has been developed an electric vehicle carrying a battery, such as a lead battery, a nickel-hydrogen battery, a lithium ion battery, and a fuel cell, and equipped with an electric motor, or a hybrid vehicle employing these batteries and an internal combustion engine in combination. In such vehicles, a transmission oil and an electric motor oil are used separately in each equipment.
It is now desired to develop an oil that can commonly be used in both the transmission and the electric motor and also desired to develop a packed system of such equipment for making electric vehicles and hybrid vehicles smaller and lighter. Therefore, it is now desired to produce an oil having insulating ability and cooling ability as an electric motor oil, in addition to lubricity as an oil for a manual transmission, an automatic transmission or a continuously variable transmission.
A transmission oil is desired to have stability against heat and oxidization, detergency-dispersancy ability, anti-wear ability, and anti-seizure ability. In order to fulfill such requirements, a transmission oil in general contains a base oil such as a mineral oil or a synthetic oil, and a variety of additives such as anti-oxidants, detergents/dispersants, anti-wear agents, rust inhibitors, metal deactivators, friction modifiers, antifoam agents, coloring agents, seal swellers, and viscosity index improvers. Such a transmission oil has low volume resistivity and insufficient insulating ability. Therefore if such an oil is used in an electric motor, that would result in troubles such as short circuit of the electric motor, insufficient cooling due to its high kinematic viscosity, and power loss upon transmitting.
On the other hand, an electric motor oil is desired to have insulating ability and cooling ability, whereas it does not need to have lubricity. Therefore the electric motor oil contains almost no additive. Employment of such an electric motor oil in the transmission system would result in severe seizure or wear trouble of bearings and gears.
That is, there has existed no transmission oil composition that has the anti-seizure and anti-wear ability as a transmission oil, as well as the insulating ability and cooling ability for automobiles especially such as the electric vehicles and hybrid vehicles equipped with the electric motor.